Locks for doors, windows and gates come in many forms from simple sliding bolts or rotating catches to modern multi-point locks including hooks, or cam rollers. Whilst the technology for locks has improved, the demands on doors have increased markedly. Doors are expected to not just close an aperture but are now expected to prevent determined intrusion and to seal against airflow in order to prevent heat loss.
In some specialist applications doors may be required to resist floodwater, blast, hurricanes or vehicles. In these cases the operation of the locking and sealing mechanisms may be very complex using worm gears, bevel gears, ratchets or other highly mechanised means. In some cases this operation may be aided by electronic means although this inevitably increases the cost and complexity of the locking apparatus significantly.
For buildings, such as dwellings, it is preferred to have inward opening doors for use as the main access door to the building. In the case of inward-opening doors, the locks are required to provide sealing compression in order to resist winds and in specialist applications to resist hydrostatic forces. This is despite the fact that the opening direction of the door is not at all advantageous as these forces will in effect be opening up the seal. This results in modern locks potentially working at the very edge of their performance capability and presents the risk of catastrophic failure.
With adjustable locks these may be tightened up to provide good compression against the forces trying to open the door or gate, however because of the load required to be applied through the handle or other means in which to operate the locks, the forces required to activate the locks may be beyond that provided in building or other regulation in some cases making it inoperable to many users. It will also potentially damage the soft seals through crushing, damage hinges or other elements through hyper-extending, and will still be vulnerable to catastrophic failure though failure of a fixing or snapping of a lock component. Handle forces required for operation and the sealing pressure required may be inextricably linked and induce an inherent weakness into the system.